Syrian hamsters are long-day seasonal breeders whose reproductive systems become quiescent in the short days of fall and winter. When inbred LSH/SsLak female Syrian hamsters are transferred from a long (LP) to a short photoperiod (SP) in the laboratory, ovulatory cycles cease. This event is mediated by the pineal gland since its presence is required for SP to produce an antigonadotropic effect. Ostensibly, diminishing photoperiods lead to the production of a pineal substance which produces this antigonadotropic effect. Although the pineal indoleamine melatonin is a strong candidate for this pineal hormone, the neuroendocrine events underlying the changes in the reproductive system in response to diminishing photoperiods have not been resolved, especially in the female hamster. It is now well recognized that pituitary gonadotropin and prolactin secretion is regulated by hypothalamic norepinephrine and dopamine turnover rates, and perhaps other neurotransmitters. Change in pituitary secretion induced by changing photoperiods and pineal activity most likely involve alterations in these controlling mechanisms. Our goal is to elucidate neuroendocrine mechanisms through which the pineal gland transduces information about changes in the environment into altered reproductive states. Our recent studies have characterized the early and comparatively homogeneous response of the inbred LSH/SsLak female hamster to SP exposure. Reductions in serum prolactin levels and uterine weights precede detectable changes in gonadotropins. We have observed that an increase in atretic antral follicles precedes the onset of the SP-induced anestrous condition in inbred LSH/SsLak females. The proposed studies will characterize hypothalamic and preoptic area catecholamine turnover rates in the intact cycling female hamster transferred from LP to SP. The hypothesis that SP exposure induces an FSH insufficiency will be tested in animals which have been unilaterally ovariectomized in SP. Other experiments will evaluate the role of ovarian estrogens in SP-induced catecholamine turnover rates. A final series of experiments will examine the relationship of the pineal gland and its hormone melatonin to photoperiod-dependent changes in the ovulatory cycle. The studies will take advantage of an established model which should continue to yield novel and important information on the neuroendocrine control of seasonal reproduction.